1. Field of Invention
Exemplary aspects of the present invention generally relate to an image fixing apparatus and an image forming apparatus, and more particularly to an image fixing apparatus using an electromagnetic induction heating system which is included in an image forming apparatus such as a printer, a copying machine, a facsimile, etc.
2. Background
A related art image fixing apparatus using an electromagnetic induction heating system reduces a temperature increase time of an image forming apparatus for a purpose of saving energy.
For example, a related art image fixing apparatus using an electromagnetic induction heating system includes a support roller (a heat roller), an auxiliary fixing roller (a fixing roller), a fixing belt which is tensed between the support roller and the auxiliary fixing roller, an induction-heating device that counters the support roller through the fixing belt and a pressing roller which is in contact with the fixing belt on the auxiliary fixing roller. The induction-heating device includes a coil member (an excitation coil) that extends in a width direction (a main scanning direction) and a core member.
The fixing belt is heated at a position where the fixing belt faces the induction-heating device. A toner image on a recording medium is conveyed to a position between the auxiliary fixing roller and the pressing roller, and heated by the fixing belt, and fixed on the recording medium. In more detail, applying the coil member with a high frequency alternate current causes a magnetic field around the coil member. An eddy current is generated near the support roller surface. The eddy current causes a heat increase using a resistance of the support roller itself. The fixing belt is heated with the increased heat, and is tensed by the support roller. In a related art image fixing apparatus using an electromagnetic induction heating system the image fixing apparatus may increase a surface temperature (a fixing temperature) of the fixing belt to a predetermined temperature with a small amount of energy and a short temperature increase time.
In another example, a related art image fixing apparatus using an electromagnetic induction heating system includes a fixing roller (a heat roller) which is made of a ferromagnetic material and an excitation coil formed around the inside and outside of the fixing roller. This related art image fixing apparatus increases a heat generating efficiency in the fixing roller.
When fixing on a small size recording medium is continuously performed using the above-mentioned related art image fixing apparatus or a driving of the image fixing apparatus is irregularly stopped by a paper jam and so on, temperature of a part or all of a fixing member, such as a fixing belt, may excessively increase.
In more detail, a general image forming apparatus is constituted so that image formation may be performed on various types of recording media with which the sizes of a width direction of the recording media differ. The recording media have different sizes in a width direction, such as an A sequence and B sequence in Japanese Industrial Standards (JIS) regulation, and irregular sizes. Even if the same A4 size sheets are used, the width lengths differ depending on feeding direction of the sheets.
When fixing on such recording media is performed using a related art fixing apparatus, heat distribution of the fixing belt in a width direction is varied and unevenness of heat may occur. For example, when a narrow width recording medium is fed, a part of the fixing belt which is in contact with the narrow recording medium decreases in temperature due to heat consumption into the recoding medium as compared with the other part of the fixing belt. This phenomenon is especially remarkable when a narrow recording medium is continuously fed.
When a temperature of a central part of the fixing belt is used as a reference temperature, although the temperature of the central part of the fixing belt is controllable to a required temperature, the other parts of the fixing belt, such as an end position, may excessively increase in temperature. Thus, when a wide recording medium is fixed, a hot offset may occur at the high temperature position. Further, when a fixing temperature at both ends of the fixing belt is higher than a heat-resistant limit, a thermal breakage may occur in the fixing belt.
To control the temperature of the whole region of the fixing belt with a temperature of an end part of the fixing belt as a reference temperature, although the temperature of the end part of the fixing belt is controllable to a required temperature, the central part of the fixing belt may excessively decrease in temperature. Thus, when a recording medium is fixed, a cold offset may occur at low temperature position.
When a paper jam occurs, an applied drive to the fixing apparatus may be stopped suddenly. In this case, a portion of the fixing belt which counters an induction-heating device may excessively increase in temperature at a cutting off time of electricity into the induction-heating device. This may cause a thermal breakage in components such as the fixing belt and the coil member of the induction-heating device.
A position of the coil part to a heat device tends to vary in the related art image fixing apparatus, and a heat efficiency of the heat device may not be stable. That is, the heat efficiency of the heat device may be changed with the position of the heat device in a magnetic field generated by the coil member. Thus, when the heat efficiency of the heat device varies, a temperature increase time may differ according to an image forming apparatus. Such a problem may notably occur when the coil member is arranged so that it faces the front or back side of the heat device.
In the above-mentioned related art image fixing apparatus, heat generation of a coil member itself increases a resistance of the coil member. This increases a heat loss of the coil member and a power consumption. Further, a covering layer of the coil member may be damaged. When the coil member is arranged near the heat device in order to improve heat efficiency, temperature of the coil member may increase higher because the coil member easily receives a radiant heat from the heat device. These problems may notably occur when the coil member is arranged so that it faces the front or back side of the heat device of a support roller. The coil member facing an outside surface of the support roller has a wide open area around it, so that it is easy to release heat. But, the coil member facing an inside surface of the support roller has a narrow open area around it, so that it is hard to release heat. This may cause those problems.
In one of the above examples, the related art image fixing apparatus of electromagnetic induction heating system includes a fixing roller (a heat roller) which is made of a ferromagnetic material, an excitation coil formed around the inside and outside of the fixing roller. This related art image fixing apparatus increases a heat generating efficiency in the fixing roller. But this related art image fixing apparatus may not control the above-mentioned excessive temperature of the heat device. A position of the excitation coil member to the heat device may vary in the background image fixing apparatus, and a heat efficiency of the heat device may not be stable. Further, high temperature of the excitation coil member may increase a power consumption.